234 AMERICAN JOURNAL OF BOTANY [Vol. 9 



ency to usefulness on the part of the parasitic organism, and by the utmost 

 tolerance on the part of the host. In many instances the nature of the 

 reaction is not constant, but varies with the progress of the host-parasite 

 relation. In this intergrading series of possible host-parasite relations, the 

 inverse relation between host vigor and parasite virulence obtains only in 

 the instances and phases where the reaction of the host to the parasite is 

 one of active antagonism; here a more vigorous host means a host of greater 

 physiological capacity to combat the progress of the invader. But when 

 the relation between host and parasite is of a symbiotic type, a more vigorous 

 host means a host in which more food is available for the development of 

 the parasite. Because, of the general class of parasitological phenomena, 

 the instances mainly in the field of pathological interest (the diseases 

 ordinarily so called) are an artificially selected group in which relations of 

 violent antagonism between host and invading organism are most promi- 

 nently in evidence, thought in the field of pathology has developed with the 

 physiological antagonism of host and parasite as its basal concept; and the 

 theories of immunity extant are largely concerned with the nature of the 

 antagonistic reactions. 



In the group of the fungi the transition from violent and destructive 

 parasitism to parasitism of^the symbiotic type is accompanied by a transi- 

 tion from facultative to obligate parasitism, as if the physiological corollary 

 of parasitism of the latter type is extreme specialization in food preferences. 

 The series in the fungi grades from violent and destructive parasites like 

 Botrytis, on the one hand, to, on the other hand, so benign an infestation as 

 the seed fungus of Lolium temulentum (described by Freeman, 1903) in 

 which the relation is so intimate and devoid of any untoward effect on the 

 host, and the life history of the cohabiting organism is so parallel with that 

 of the grass that its distinct individuality is almost open to question. 



The mutualistic nature of the relation between host elements and 

 fungus in rusts of the type of the cereal rusts is commented on by Tubeuf 

 (1897, p. 91) who very aptly compares the mass of chlorophyll-bearing 

 leaf cells infested with the rust mycelium to a lichen structure, especially 

 to those lichens whose algae obtain water and inorganic materials direct, 

 rather than through the fungous hyphae. Certainly, during the greater 

 part of the relation, there is here no evidence of any deleterious effects on 

 the host cells. While the contribution of the affected elements to the 

 .growth and fruiting economy of the host plant as a whole may be diminished, 

 the infected protoplasts continue essentially unimpaired in structure and 

 function. The parasite does not attack the living substance of the host 

 protoplast, but confines itself to establishing such a relation with the latter 

 that it shares the available food resources of the cell ; and the rust haustorium 

 is not an implement for mechanical disruption, but a structure more in the 

 nature of the placenta of the mammalian foetus for establishing physiological 

 communication with the food resources of the host. 



